Observations of Fatigue Damage in the Press-Fitted Shaft under Bending Loads

Abstract:

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In this paper, the characterization of fretting damage on press-fitted specimens is
proposed by experimental methods. A series of fatigue tests and interrupted fatigue tests on pressfitted
specimens were carried out by using a rotate bending fatigue test machine. Macroscopic and
microscopic characteristics were observed to identify fretting damage mechanism with a scanning
electron microscope (SEM) and profilometer. The mechanism of fretting fatigue damage on pressfitted
structure is discussed from experimental results. It is found that small cracks of 30~40m in
depth are initiated when the specimen reached about 10% of the total life, and thus almost 90% of
the fretting fatigue life of press fits can be considered to be in the crack propagation phase. Most of
fatigue cracks are initiated at 1050m inner side of contact edge, and multiple cracks are
nucleated and interconnected in the fretted surface. The crack nucleation angle in the near contact
edge region is larger than that in the inside of the contact edge region. The fretting wear increased
with increasing fatigue cycle. Since the fretting wear is relevant to the evolution of surface profile,
the fretting fatigue is observed to be closely related with the fretting wear.

Abstract: A two-parameter constraint-based fracture mechanics approach is used to explain the effect of the constraint on the apparently anomalous behavior of short fatigue cracks. The different levels of stress constraint are quantified by the T-stress, and microstructurally as well as mechanically short cracks are discussed. Short cracks generally behave more sensitively to the constraint than the long ones. It is shown that in most cases, the existence of short cracks goes hand in hand with an intrinsic loss of the constraint, which contributes to a decrease of their fatigue
threshold values and accelerates their growth. In this paper, the above effect is quantified and conclusions concerning the applicability of the fracture mechanics parameters and approaches to the estimation of the residual fatigue life of structures are discussed.

Abstract: The effect of undissolved ferrite amount on impact fatigue properties and failure mechanism
were studied by using 42CrMo steel with subcritical quenching process The amount of undissolved
ferrite were 0%, 10% and 15%, respectively. The experimental results show that the existence of
undissolved ferrite can not only change the microstructure, but also increase the impact fatigue life The
impact fatigue life elongates with increasing of amounts of undissolved ferrite The grain can be fined by
using subcritical quenching process and the area of phase boundaries can also be greatly increased
because of undissolved spheroidal carbide. The martensite and carbide form can also be changed by
using subcritical quenching process The stress relaxation due to the moving of dislocations inside the
ferrite and the promotion of strength due to occurring of plastic deformation and the enwinded
dislocations are main reasons of improving the impact fatigue life. The impact fatigue life elongates
with the increase of amounts of undissolved ferrite before the amount of undissolved ferrite reaches 10%.
Under the experiment conditions, when the amount of undissolved ferrite is 10%, the impact fatigue life
will be the longest.

Abstract: In this research, corrosion fatigue tests using tensile strength of 490MPa TMCP steel
were performed in synthetic seawater condition to investigate the corrosion fatigue crack
propagation characteristics. The influence of cathodic protection at -800mV vs. SCE on the
corrosion fatigue crack propagation behavior was investigated. Relationships between da/dN versus
ΔK for the material were obtained by two types of test specimens. In the present study, F(α,β)
versus α(= 2a/W) relationship for the CCT specimen was calculated by J integral approach, while
that for the CT specimen was determined from ASTM E647. It is found that the fatigue crack
propagation rate of TMCP steel in synthetic seawater condition is faster than that in air condition at
least twice. Also, it is observed that the fatigue crack propagation rate of steel with cathodic
protection is in between those of seawater condition without cathodic protection and air condition.

Abstract: The formulations of fatigue crack growth prediction are still mostly based on
phenomenological models. A commonly used formula in the field of high cycle fatigue is the Paris-
Erdogan law. For given experimental conditions (such as temperature, stress ratio or environmental
conditions) the parameters C and m have to be experimentally determined and considered as
material constants. Thus, for a given material, the fatigue crack growth rate (FCGR) depends only
on the applied range of the stress intensity factor. In a threshold region a significant shift in the data
of the fatigue crack propagation rate can be observed. The shift is induced by different test specimen
geometry. To analyses it the authors will present their own laboratory fatigue crack growth rate test
data measured on two different specimens with different levels of constraint and for different steels.
It is demonstrated that fatigue characteristics (i.e. C, m and Kth) obtained from different specimen
geometries are not only properties of the materials but depends on the specimen geometry.